When laying a pipeline at sea one of two methods is commonly used: either the “S” laying method or the “J” laying method. The two methods are named in accordance with the general shape adopted by the pipeline during laying. In “S” laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. In “J” laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. As interest in laying pipeline in deep water has increased, so “J” laying has become more attractive because the pipeline naturally adopts a vertical or near-vertical orientation far from the seabed. “J” laying is not, however, preferred in shallower water where the natural path of the pipeline is only ever inclined at a shallow angle as it passes to the seabed and “S” laying is therefore advantageous.
When designing a vessel it is in principle possible to equip it for both “J” laying and “S” laying and then to choose which laying technique to use in any particular case. That is, however, an unsatisfactory solution because of the size and cost of the equipment required. Another solution that has been proposed is to provide a “J” lay vessel in which the “J” lay tower can be pivoted to any of a variety of angles according to the depth of the water. A problem with that solution is that the same tower has to be able to be operated over a wide range of angles, which, for example, makes it difficult to provide welding stations on the tower.
“S” laying can be employed in deep water provided the pipeline being laid can be supported from the vessel until it has reached a relatively steep inclination to the horizontal. That, however, requires a considerable length of support, because the radius of bending to which the pipeline can be subjected is limited, especially in the case of large diameter pipelines.
One approach to providing an “S” laying vessel includes providing a semisubmersible vessel with twin keels. For example, U.S. Pat. No. 4,257,718 shows such an arrangement. Another approach to providing an “S” laying vessel involves providing a monohull vessel as shown, for example, in U.S. Pat. No. 5,823,712. An advantage of a twin keel semisubmersible vessel is that it can provide a relatively roll-free environment for the pipe laying and also considerable amounts of space to either side of a central pipe laying path on the vessel (also known to those skilled in the art as “the firing line”). The extra breadth of such a vessel and its relatively large draught are, however, disadvantages compared to a monohull vessel.
In order to benefit from having a monohull vessel, it is desirable to keep the dimensions of the vessel as small as possible, while providing the necessary facilities. When laying small diameter pipes it is sometimes preferred to provide a long length of prefabricated pipeline on a reel and to lay the pipeline by unwinding the reel, but especially for large diameter pipelines it is usual to form the pipeline from discrete lengths of pipe each typically about 12 m long. In this case it is desirable to weld individual lengths of pipe into prefabricated pipe lengths (also referred to herein as “jointed pipe sections”) consisting of, say, two, three or four individual lengths of pipe and then to weld the prefabricated pipe lengths to the end of the pipeline as it is being laid. Such a procedure enables the speed of laying to be increased over that which would apply if each individual pipe length were welded in turn to the end of the pipeline as it is laid. Thus the vessel is required to accommodate welding stations and other facilities for preparing prefabricated pipe lengths.
A monohull vessel is susceptible to rolling when at sea and, in order to reduce the effect that the rolling action has, it is desirable to have the welding station(s) at which a new prefabricated pipe length is added to the pipeline being laid close to the axis of rolling; the rolling action then causes only a little oscillating movement at the welding stations.
A monohull vessel of this kind is shown in U.S. Pat. No. 5,823,712. The vessel has a horizontal pipelaying path in the region of the rolling axis of the vessel and this path extends substantially the whole length of the vessel and accommodates at the upstream (bow) end the necessary welding stations for welding new prefabricated pipe lengths to the pipeline and at the downstream (stern) end tensioners for tensioning the pipeline between the vessel and the seabed. In order to provide controlled bending of the pipeline away from the horizontal a “tail unit” is provided that projects from the stern of the vessel hull and that is of a length comparable to that of about half the length of the vessel hull. The pipeline therefore leaves the stern of the vessel hull at an angle only gently inclined to the horizontal and the tail unit provides a long extension to the vessel.
The arrangement just described is useful in providing a lay path along which to first weld, then tension and then bend the pipeline but has the disadvantages that the part of the pipeline being laid over the tail unit is exposed (which is a particular problem in icy waters) and that the load applied to the vessel by the pipeline is applied at the end of the tail unit which is much further from the barycentre (centre of gravity) of the vessel than the stern of the vessel hull and can therefore apply large moments (torques) to the vessel.
It is an object of the invention to provide a monohull vessel for laying a pipeline and a method of laying a pipeline which overcomes or mitigates at least some of the disadvantages described above.